Damper

ABSTRACT

An extrusion damper for interposing between two members to absorb energy of motion which may be induced between the two comprises an outer jacket ( 1 ) surrounding a body of plastically deformable energy absorbing material ( 3 ) such as lead and a shaft ( 2 ) which moves through the outer jacket to deform the lead during induced motion. The body of lead may be fixed relative to the outer jacket and the shaft has a portion of reduced diameter ( 2   a ) which moves through the lead. Alternatively the body of lead may be fixed to the shaft and the outer jacket has a portion of enlarged diameter ( 21   a ) through which the energy absorber passes. The body of energy absorbing material may optionally be subjected to approximately hydrostatic pressure preferably exceeding the shear yield stress of the energy absorbing material.

This invention relates to dampers of the type commonly referred to asextrusion dampers, used for reducing the effects of induced motion ordisplacement in a variety of structures and equipment.

The dampers of the invention may be used in large structures such asbridges or buildings to reduce the effects of motion induced duringearthquakes or from strong winds. They may also be used to damp motionof large or small moving objects. They may be used to damp motion inindustrial machinery or engines or the like or from domestic appliancessuch as washing machines for example, or in any other application whereit is desired to damp any motion, vibrations or similar. They may beused to damp displacement arising from thermal expansion. The extrusiondampers of the invention have various applications.

BACKGROUND OF THE INTENTION

Devices known as extrusion dampers which employ elastic or plasticdeformation of certain materials to absorb energy are well known. U.S.Pat. No. 3,833,093 describes a form of extrusion damper consisting of anenergy absorber material confined between an elongate outer jacket whichis typically a cylinder, and a shaft which moves longitudinally withinthe jacket. The absorber material is typically lead while the jacket andshaft are typically formed of steel. Opposite ends of the jacket andshaft are connected between two members in a structure which areexpected to move relative to one another during an earthquake or otherinduced motion. A general discussion of these and related devices isgiven in “An Introduction to Seismic Isolation”, R I Skinner, W HRobinson and G H McVerry, Wiley, 1993.

Lead is the preferred deformable energy absorbing material for severalreasons. First it yields at a room temperature shear stress of around10.5 MPa which is low compared with other metals and equivalent plasticmaterials. Second it restores its mechanical properties throughrecrystallisation and associated processes relatively rapidly followingyield deformations, which provides outstanding resistance to workhardening under cyclic shear at ordinary temperatures. Third lead isreadily available at the purity required to exhibit these properties.

In practice such dampers are individually designed to protect aparticular structure against damage by damping certain motions impartedto it. Their behaviour is quite closely approximated by that of an idealCoulomb damper in having a force-displacement hysteresis loop which isnearly rectangular and practically rate independent over a wide range offrequencies. Research into performance of these devices is ongoing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide for improvedperformance of such dampers for seismic isolation and otherapplications.

The invention broadly comprises a damper for interposing between twomembers to damp motion which may be induced between the two, comprisingan elongate outer jacket means, a shaft passing through the outer jacketwhich is forced to move through the outer jacket during induced motionand a body of a plastically deformable energy absorbing material fillingthe space between the outer jacket and the shaft, wherein the body ofenergy absorbing material is fixed relative to the outer jacket and theshaft comprises a portion of reduced diameter which is forced throughthe energy absorbing material during said induced motion and/or the bodyof energy absorbing material is fixed to the shaft and the outer jacketcomprises a portion of enlarged diameter through which the body ofenergy absorbing material is forced during said induced motion.

Preferably the body of energy absorbing material is subjected toapproximately hydrostatic pressure at least approaching the shear yieldstress of the material. Preferably the hydrostatic pressure applied tothe energy absorbing material exceeds the shear yield stress of theenergy absorbing material. Preferably the hydrostatic pressure is 5 MPaor more and most preferably in the range 10-100 MPa.

Preferably the energy absorbing material is lead, but other energyabsorbing materials which may be used include alloys of lead, aluminiumat elevated temperature e.g. about 200° C., tin, zinc, brass, iron,super plastic alloys, or any other material having a low rate of workhardening, including also densely packed granular materials such assteel shot, glass beads, alumina, silica, silicon carbide or any othervery hard granular material.

Dampers of the invention may be used in seismic isolation applicationsto damp seismic motion in large structures such as bridges or buildingsor motion from very strong wind buffeting or similar. They may also beused in any other application where it is desired to damp any motion,vibrations, or similar. For example, dampers of the invention may beused to damp motion of engines or other industrial machinery. Indomestic applications, dampers of the invention may be used in washingmachines or spin dryers or dish washers to isolate vibrations. Smallsize dampers of the invention may be used as “microisolators” forsensitive electronic equipment such as the mechanism of a video recorderetc or in other similar applications. Numerous applications of theextrusion dampers of the invention are envisaged and the invention isnot limited only to seismic isolation dampers.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred dampers of the invention will be described by way of examplewith reference to the following drawings, wherein:

FIG. 1 is a perspective view of one form of damper of the invention inlongitudinal cross-section,

FIG. 2 shows the damper of FIG. 1 in longitudinal cross-section,

FIG. 3 shows another form of damper of the invention similar to that ofFIGS. 1 and 2 in longitudinal cross-section,

FIG. 4 shows another form of damper of the invention also similar tothat of FIGS. 1 and 2 in longitudinal cross-section,

FIG. 5 shows a further form of damper of the invention again similar tothat of FIGS. 1 and 2 in longitudinal cross-section,

FIG. 6 shows another form of damper of the invention in longitudinalcross-section,

FIGS. 7 and 8 are Mohr circle constructions which will be referred tofurther in description of the extrusion dampers of the invention,

FIG. 9 is a graph of shaft displacement against time graphicallyillustrating testing applied to a damper of the type shown in FIGS. 1 to5,

FIG. 10 is a graph of load resistance exhibited against time for adamper of the type shown in FIGS. 1 to 5 subjected to the displacementcycling of FIG. 9, and

FIG. 11 is a graph of load resistance exhibited during the test cyclingof FIG. 9 against displacement showing successive hysteretic loops forsuccessive cycles.

DESCRIPTION OF PREFERRED FORMS

The dampers shown in FIGS. 1 to 5 each comprise an outer jacket 1 whichis typically formed of steel and may be cylindrical as shown, but couldbe of other cross-sectional shapes such as oval for example. A shaft 2is able to move longitudinally through the outer jacket 1 in thedirection of arrow A in each case. It is not necessary for the shaft tobe positioned centrally within the outer jacket 1, but it could insteadbe off-set somewhat. The shaft is also typically formed of steel.

The space between the outer jacket 1 and shaft 2 is filled with aplastically deformable material such as lead so that in use the shaftmoves through the lead.

The shaft 2 moves through end cap 4 which also forms a bearing for oneend of the shaft and bearing 5 on the other side which floats. A tubularspacer 6 is provided between an end cap 7 and the floating bearing 5.

A central part shaft 2 of the shaft 2 a has a reduced diameter as shown.FIG. 1 shows the damper from the exterior while FIGS. 2 to 5 show thedamper in longitudinal cross-section showing variations in constructionwhich will be referred to further later.

In use the outer jacket 1 of the damper is coupled to one member of abuilding or other structure through a suitable mechanical coupling, andthe shaft is coupled to another member, through the end 2 b of theshaft, which may move relative to the first in an induced motion. Duringsuch motion such as an earthquake in a seismic isolation application,the shaft 2 and in particular the reduced diameter part 2 a is forcedthrough the deformable material 3 such as lead, which creates a dampingeffect by conversion of kinetic energy to plastic deformational energyand heat, and further heat during recrystallization and otherspontaneous recovery processes.

The energy absorbing material may optionally be prestressed under anapproximately hydrostatic pressure at least approaching and preferablyexceeding the shear yield stress of the material so that the materialwill always be in compression. With lead pressures of 5 MPa or more,typically 10 MPa to around 30 MPa, but also up to 100 MPa or more havebeen found effective.

The effect of a hydrostatic pressure may be explained briefly by way ofthe Mohr circle constructions shown in FIGS. 7 and 8, which enables theproperly tensor description of stress to be represented in twodimensions. A hydrostatic pressure applied to a body is then defined asone third the sum of the three principal stresses which act upon it. InFIG. 7 the hydrostatic pressure is 0, and the principal tensile stressσ_(x), the principal compressive stress σ_(y) and the maximum shearstress σ′_(xy) are all equal in magnitude. In FIG. 8 a hydrostaticpressure p equal to the shear stress σ′_(xy) has been applied. Themaximum tensile stress is then 0 so that the body is always undercompression. Therefore the body cannot fail in tension.

Returning to FIGS. 2 to 5 a number of alternative arrangements forapplying hydrostatic pressure to the lead 3 (or other energy absorbingmaterial) are shown. In FIG. 2 a block comprising layers of anelastomeric material 8 such as rubber and rigid material 9 such as steelis provided between the tubular spacer 6 and the end cap 7. In FIG. 3 apad 10 of elastomeric material such as rubber is provided at either endof the absorber material 3. In both cases the length of the spacer 5 issuch that when the end cap 4 is screwed home, the desired hydrostaticpressure is applied to the lead 3. In FIG. 4 the exterior of the body ofabsorber material 3 is surrounded by a sleeve of elastomeric materialsuch as rubber or reinforced rubber or other resilient material. In FIG.5 the absorber material is entirely encased in such a layer ofelastomeric material 12.

In each case the energy absorbing material 3 may be cast directly intoplace within the outer jacket 1 and around the shaft 2. In theembodiments of FIGS. 4 and 5 the rubber sleeve or casing may bestretched around the lead body after casting around the shaft and theshaft and lead then press fitted into the outer casing. Alternativelythe lead may be cast as a plug with a constant diameter bore through thelead. The lead plug may then be inserted into the outer casing and theshaft then pressed into the central bore through the lead. Pressure maythen be applied to the lead, for example by fitting of the end cap 4 orequivalent, to compress the lead to cause the lead to move to surroundthe reduced diameter part 2 a of the shaft completely.

As stated the damper may be cylindrical or alternatively oval, square,rectangular or any other desired shape in overall cross-sectional shape.

The shaft and also other parts of the damper may optionally be coatedwith teflon, porcelain, titanium nitride, a hard ceramic material,glass, or similar.

Preferably during assembly of the dampers the component parts are coatedwith a high temperature/pressure grease or other lubricant.

FIG. 6 shows another type of damper of the invention. Again the dampercomprises an outer jacket 21 typically formed of steel which isdesirably cylindrical but could be of other cross-sectional shapes. Onepart of the outer jacket 21 is formed with a portion 21 a of enlargeddiameter as shown. A shaft 22 which is enlarged at either end 22 apasses through the outer jacket 21. The ends 22 a of the shaft formplungers within the outer jacket 21. Seals 26 such as chevron seals maybe provided to seal the plunger parts 22 a of the shaft against theinternal bore of the outer jacket. Parts of the plunger ends 22 a of theshaft 22 are shown in cross-section. The end of the shaft extends out ofthe outer jacket further than shown in the drawings on at least oneside, to enable the shaft to be coupled into position in use. The shafthas a reduced diameter centre part 22 b. In the drawings the shaft isshown as a three part shaft with the reduced diameter centre part 22 athreading into the larger diameter parts 22 b of the shaft at eitherend, as indicated at 22 c.

A body of lead 23 or other energy absorbing material is fixed around thereduced diameter part 22 b of the shaft 22. The lead may be subjected tohydrostatic pressure by screwing the plunger ends 22 a onto the centreshaft part 22 b to apply the desired pressure to the lead, which is alsoconfined within the outer jacket 1. Other arrangements are possible.

In use the outer jacket 21 is coupled to one member of a building orother structure, and one or both ends of the shaft 22 are attached toanother member which may move relative to the first in an inducedmotion. During such motion such as an earthquake in a seismic isolationapplication, the shaft 22 moves relative to the outer jacket 21, as itdoes so forcing the body of lead 23 through the interior of the outerjacket 21 including the enlarged diameter portion 21 a in the outerjacket, creating a damping effect by conversion of kinetic energy toplastic deformational energy, and to further heat duringrecrystallisation and other spontaneous recovery processes.

Again the shaft and also the interior of the outer jacket may be coatedwith teflon, porcelane, titanium nitride etc and the component parts ofthe damper may be coated with a high temperature grease during assembly,as referred to previously.

The following test of a damper of the invention further illustrates theinvention:

TEST

A damper was constructed comprising a cylindrical outer jacket of steelof internal diameter 40 mm. The shaft was a cylindrical steel shaft of19 mm diameter with a central portion 20 mm long in which the diameterof the shaft reduced smoothly to 12 mm minimum diameter and thenincreased back to the normal diameter of the shaft. Lead of 99.9% puritywas cast into place between the outer jacket and the centrallypositioned shaft to surround the shaft within the outer jacket. Thelength of the lead slug fixed within the casing and surrounding theshaft was 130 mm. Before casting the lead into place the shaft wascoated with a high pressure lubricant. When the end cap of the outerjacket was screwed home the lead was subjected to approximately 20 MPahydrostatic pressure. An Instron testing machine subjected the damper tocycles of shaft movement of displacement of ±195 mm with a maximum crosshead speed of 200 mm/minute and a maximum force of 250 kN. The resultswere recorded directly on a chart recorder connected to the Inston andby a data logger. FIG. 9 shows shaft displacement against time. FIG. 10shows the load resistance exhibited by the damper, against time. FIG. 11shows the load resistance exhibited by the damper against displacement,showing successive hysteretic loops for successive cycles. Afterextended testing the damping force and energy absorbed per cycle werestill within 20% of the starting values. At the completion of extendedtesting the damper was removed from the test rig and disassembled. Thelead was visually inspected and found to be in good condition.

The foregoing describes the invention including various preferred formsthereof. Alterations and modifications as will be obvious to thoseskilled in the art are intended to be incorporated herein as defined inthe claims.

What is claimed is:
 1. A damper for interposing between two members todamp motion which may be induced between the two, comprising an elongateouter jacket, a shaft passing through the outer jacket which is forcedto move through the outer jacket during induced motion, and a body of anenergy absorbing material substantially completely filling the spacebetween the outer jacket and the shaft, wherein either the body ofenergy absorbing material is fixed relative to the outer jacket and theshaft includes a maximum diameter within the jacket and an operativeportion of reduced diameter relative to said maximum diameter and formedby an arcuate wall portion having a radius of curvature greater thansaid maximum diameter, said energy absorbing material being inflexibleand normally solid but damping movement of the shaft through the energyabsorbing material occurs when the centre portion of the shaft is forcedto move through said energy absorbing material during said inducedmotion, by a process of plastic deformation and recrystallisation of theenergy absorbing material, or the body of energy absorbing material isfixed to the shaft and the outer jacket comprises an operative portionof an enlarged diameter relative to the diameter of the outer jacket oneither side of said operative portion of the outer jacket, said energyabsorbing material being forced through said operative portion of theouter jacket during said induced motion, said energy absorbing materialbeing inflexible and normally solid but damping movement occurs when theshaft and the body of energy absorbing material fixed thereto are forcedto move through said operative portion of the outer jacket during saidinduced motion by a process of plastic deformation andre-crystallisation of the energy absorbing material.
 2. A damperaccording to claim 1, wherein the body of energy absorbing material issubjected to approximately hydrostatic pressure at least approaching theshear yield stress of the energy absorbing material.
 3. A damperaccording to claim 1, wherein the body energy absorbing material issubjected to hydrostatic pressure exceeding the shear yield stress ofthe energy absorbing material.
 4. A damper according to claim 3, whereinthe energy absorbing material is formed of lead.
 5. A damper accordingto claim 3, wherein the shaft and/or the interior of the outer jacket iscoated with teflon, porcelane, titanium nitride, or other ceramicmaterial, or glass.
 6. A damper according to claim 1, wherein the bodyof energy absorbing material is subjected to hydrostatic pressure of 10MPa or more.
 7. A damper according to claim 1, wherein the body ofenergy absorbing material is subjected to hydrostatic pressure exceeding20 MPa.
 8. A damper for interposing between two members to damp motionwhich may be induced between the two, comprising an elongate outerjackets a shaft passing through the outer jacket which is forced to movethrough the outer jacket during induced motion, and a body of an energyabsorbing material substantially completely filling the space betweenthe outer jacket and the shaft, wherein either the body of energyabsorbing material is fixed relative to the outer jacket and the shaftis of substantially constant diameter within the jacket except for asingle operative portion of reduced diameter, or the body of energyabsorbing material is fixed to the shaft and the outer jacket comprisesan operative portion of an enlarged diameter relative to the diameter ofthe jacket on either side of said jacket operative portion.
 9. Thedamper of claim 8, wherein said single operative portion of reduceddiameter is defined by an arcuate wall portion.